Fiber Channel Arbitrated Loop (FC-AL) protocol provides simple, low-cost connectivity to multiple Fiber Channel (FC) devices without the expense of a fully switched fabric. The standard definition of an Arbitrated Loop allows for up to 126 participating node ports to communicate with one another in a single loop.
Three major Fiber Channel topologies are illustrated in FIGS. 1A-1C. An example of Arbitrated Loop as defined within the Fiber Channel standards is illustrated in FIG. 1A. A point-to-point topology, as illustrated in FIG. 1B, is the simplest topology but also the most limited. A switched fabric topology, as illustrated in FIG. 1C, provides interconnection for thousands of nodes and manages these connections but with high overhead. Arbitrated loop provides interconnection for up to 126 node ports without adding the complexity of the centralized switching function, since the switching function has been distributed to each of the nodes on the loop. The loop is a shared media between the ports connected to it. Ownership of the loop is arbitrated for among all the loop ports, after which the winning port gets to singly use the bandwidth of the loop.
The physical implementation of an arbitrated loop requires that all drives be connected together in a daisy chain fashion with the end of the chain connecting back to the beginning of the chain. Connecting drives directly to other drives can be problematic. The loop can become non-operational when inserting or removing a drive, and also if a drive or a connection to a drive becomes faulty. A connectivity device is commonly used within each enclosure of drives that completes the connections between the loop devices. FIGS. 2A and 2B illustrate loop configurations with different types of connectivity devices. FIG. 2A illustrates loop devices attached to a loop hub/port bypass controller 201. FIG. 2B illustrates loop devices attached to a loop switch device, such as a loop switch 211.
More node ports may be added to the loop beyond the limit of 126, but these ports cannot participate in the loop operations or communicate with other ports since they will not be able to acquire an address. To allow greater than 126 node ports in arbitrated loop, more than one arbitrated loop must be created. In the example illustrated in FIG. 3, each loop 301 and 302 connects to a switched fabric 303 through fabric ports 304 and 305, respectively. This arrangement extends the addressing range, but also increases cost.
To avoid the cost of adding a switched fabric to a low cost arbitrated loop, the addressing scheme needs to be extended. Also, the loop protocol that is used to create the connection between node ports will need to be enhanced at certain levels to utilize the extended addressing scheme.
Some advances regarding latency have made it possible to have more nodes without reaching saturation. However, in most cases there has not been a need to extend FC-AL addressing, since many customers saturate a loop with 70 drives and do not even use the full capacity of 126 addresses. As newer methods evolve to increase performance in the back-end of Fiber Channel storage arrays, there will be a desire to increase the number of drives within these arrays. Current solutions for increasing drive count within an FC-AL topology are costly and include adding a switched fabric, which also adds overhead.
It is, therefore, desirable to provide a scheme to extend FC-AL addressing that overcomes at least one drawback of previous approaches.